This invention is for the purpose of providing a more sensitive and reliable mechanism for determining the precise level of the surface of a bath of molten glass in a furnace or tank holding the glass. It is desirable to know the level of the glass in a melter, either for indicating purposes, or for the purpose of controlling the addition of batch ingredients to the tank so as to maintain a fairly constant supply of glass in the melter. Melters used for supplying glass to bottle-forming machines are of the continuous melting type, where batch ingredients are added to the surface of the molten glass at one end of the melter, with the totally melted glass exiting from an opposite end. This end of the melter connected to a refiner and forehearth from which glass, in the form of gobs, will be sent to machines which form them into bottles. Most modern glass plants operate 24 hours a day, 7 days a week, thus different shifts of personnel will be in charge of the melters and feeding of glass to machines.
It is important in order to produce constant good glass, too, that the level of glass is being generally maintained within closely defined limits. This is most certainly true with regard to the glass in the feeder since the weight and size of the gobs formed by the feeder are definitely influenced by the depth of the glass in the feeder. A prior art example of a glass level gauge is U.S. Pat. No. 2,628,428 dated Feb. 17, 1953. In this patent, a water-cooled electric probe is moved into and out of contact with the molten glass under the control of a reversible motor. The speed of the motor is relatively constant while driving the probe into contact with the glass or when moving the probe away from the glass.
Another example of a glass level gauge may be found in U.S. Pat. No. 2,645,749 dated July 14, 1953. In the system described in this patent, two probes are permanently mounted at different levels and extend through the wall of the glass tank. The probes are set with their lower ends at slightly displaced vertical positions. Sensing of the level of glass by one or both probes results in the completion of electrical circuits so as to fire one or both thyratrons which, in turn, effectively operate relays. The relays operate switches which control the operation of one or more batch feeders.